The present invention relates to a color cathode ray tube, and relates more particularly to a color cathode ray tube that suppresses deformation of the color selection electrode, of which a shadow mask is typical.
Color cathode ray tubes (xe2x80x9cCRTxe2x80x9d below) such as the shadow mask type color CRTs used in color televisions and color display monitors for office automation equipment have an approximately rectangular panel portion, a substantially cylindrical neck portion housing electron guns, and a substantially funnel-shaped funnel portion connecting the neck and panel portions to form a vacuum vessel, and a shadow mask fixed to a mask frame inside the vacuum vessel. The panel has a phosphor screen disposed on the inside surface thereof with numerous red (R), green (G), and blue (B) phosphor pixels arrayed in a dot or striped pattern. The shadow mask, which is a color selection electrode and is disposed in proximal opposition to the phosphor screen, has numerous electron beam apertures.
Thin metal sheets have been used to make the shadow mask in recent years as color CRT resolution has improved. Mild steel, invar, and other metals have been used for the shadow mask.
The shadow mask is manufactured by etching the numerous electron beam apertures into the thin metal sheet in specific positions, stamping the metal sheet to a specific outside shape, and then in a press shaping the sheet to form a curved main screen part and a skirt part that is contiguous to and bent approximately 90 degrees to the main screen part. The shaped shadow mask is then fastened to the mask frame to form a mask assembly.
So-called spring back occurs in the skirt part of this press-shaped shadow mask, thus causing the skirt part to warp to the outside, that is, in the direction away from the CRT axis. If the skirt is fastened to the mask frame to form the mask assembly with this spring back (warping) remaining in the skirt, deformation in the main surface of the shadow mask occurs as a result of this skirt warping, thus degrading CRT image quality.
Various measures have been conventionally used to prevent such deformation of the main shadow mask surface. Japanese Utility Model Laid-open No. 95353/1977, for example, teaches technology for disposing a strength adjusting part to the shadow mask skirt. Japanese Patent Laid-open No. 81444/1980, for example, teaches technology for imparting surface roughness to the inside surface of the shadow mask skirt as a means for scattering electron beams. Furthermore, Japanese Patent Laid-open No. 47649/1992 teaches technology for disposing hemispherical recesses of a specific diameter and depth to the outside of the shadow mask skirt.
Yet further, Japanese Patent Laid-open No. 112566/1974 teaches technology for locally thinning a peripheral part of the main surface of the shadow mask. In addition, Japanese Patent Laid-open No. 271849/1988 teaches technology for determining the length of the shadow mask skirt to a specific value relative to the outside diameter of the panel, and fastening the tab to the mask frame by means of tabs disposed to the skirt protruding from the skirt substantially parallel to the CRT axis in the direction away from the main surface. Yet further, Japanese Patent Laid-open No. 169847/1989 teaches technology for disposing numerous substantially circular holes in the skirt at the corners of the shadow mask. Yet further, Japanese Patent Laid-open No. 35657/1997 teaches technology for forming a plurality of stress-absorbing through holes 62. Yet further, Japanese Utility Model Laid-open No. 96250/1987 teaches technology for thinning by disposing non-through holes and grooves from the edges of the main surface to the skirt part.
In addition, the technology taught in Japanese Patent Laid-open No. 271849/1988 for disposing tabs protruding from the skirt in the direction substantially parallel to the CRT axis and away from the main surface, and fastening these tabs to the mask frame, is also taught in Japanese Utility Model Laid-open No. 5657/1973 and Japanese Patent Laid-open No. 73970/1974, 72545/1990, and 22048/1992 for preventing electron beam landing misses on the phosphor screen in conjunction with thermal expansion of the shadow mask.
With the technologies noted above for alleviating spring back by disposing a strength adjusting member to the shadow mask skirt, and for disposing hemispherical recesses of a specific diameter and depth to the outside of the shadow mask skirt, however, the drop in the bending rigidity of the skirt part is insufficient. Furthermore, the conventional technologies for thinning the skirt from the edges of the main mask surface and providing through-holes only in the skirt also insufficiently alleviate spring back, and the problem of main mask surface deformation occurring easily due to warping of the skirt when the radius of main mask surface curvature is great remains.
Furthermore, it is difficult to alleviate spring back in the skirt with the technology taught in Japanese Patent Laid-open No. 271849/1988 for disposing tabs protruding from the skirt in the direction substantially parallel to the CRT axis and away from the main surface, and fastening these tabs to the mask frame. Another problem is that the tabs affixed to the mask frame are displaced and cause deformation of the main mask surface in the heating steps of the color CRT manufacturing process. It should be further noted that this deformation of the main mask surface in such heating processes is also a problem in the other technologies cited above.
Therefore, a typical object of the present invention is to provide a color cathode ray tube that resolves the aforementioned problems and suppresses deformation of the color selection electrode, of which the shadow mask is typical.
A typical configuration of a color CRT according to the present invention has a main surface with a plurality of electron beam apertures and a substantially rectangular color selection electrode with a skirt part bent substantially perpendicularly to the main surface. Bending strength in the circumference direction of the skirt part differs from bending strength in the direction parallel to the tube axis due to a buffer part formed in the skirt part. This buffer part is formed from a plurality of recessed parts different in shape from the electron beam apertures. In addition, bending strength in the circumference direction is greater than bending strength in the direction parallel to the tube axis. The recessed parts are plural substantially rectangular grooves, the long side of which is aligned with the circumference direction of the skirt. The recessed parts can alternatively be substantially circular recesses, in which case the pitch therebetween in the circumference direction differs from the pitch in the tube axis direction.
Deformation of the main surface is avoided with this configuration when the skirt is press fit to the mask frame because rigidity (i.e., resistance) to the force applied to the skirt part is different in the circumference direction and the direction parallel to the tube axis.
A high resolution display can also be achieved because the plural electron beam apertures formed in the color selection electrode are substantially circular and a dot type phosphor layer is disposed. Furthermore, because there is at least 3 mm from the neck side edge of the skirt to the buffer, the color selection electrode can be pressed to form a shadow mask of desired shape without producing cracks in the skirt.
Yet further preferably, a first tab part is disposed on the skirt of the color selection electrode protruding from the skirt in a direction away from the main surface, and this first tab part is fastened to the mask frame. The skirt of the color selection electrode can thus be easily pressed to the mask frame. Deformation of the main surface due to displacement of the tab is reduced by providing one or a plurality of through holes in this first tab part.
Yet further preferably, a second tab part is formed on a corner part of the color selection electrode protruding in a direction away from the main surface. Main surface deformation is further reduced by fastening this second tab part to the mask frame.